From the aspect of protecting global environment, a ceramic honeycomb filter (hereinafter referred to simply as “honeycomb filter”) constituted by a ceramic honeycomb structure (hereinafter referred to simply as “honeycomb structure”) having both ends on exhaust gas-introducing and -exiting sides sealed alternately is used to remove carbon-based particulates from an exhaust gas discharged from diesel engines.
In a conventional ceramic honeycomb filter 50 shown in FIG. 5, an exhaust gas containing particulates flows into flow paths 57 open at an inlet 51a, passes through cell walls 56 constituted by porous ceramics, and exits from an outlet 51b via adjacent flow paths. During this process, particulates in the exhaust gas are captured by pores of the cell walls 56. As particulates are continuously captured in the honeycomb filter 50, the pores of the cell walls 56 are clogged, resulting in drastic decrease in a capturing function and thus increase in a pressure loss and decrease in engine power. In view of this, a technology was proposed to burn particulates accumulated in the honeycomb filter 50 by an electric heater, a burner, a microwave-generating means, etc. to regenerate the honeycomb filter 50.
However, when particulates captured in the conventional honeycomb filter are burned by an electric heater or a burner, only small amounts of particulates are attached in an upstream region, so that heat generated by the burning of particulates is not sufficient to keep the self ignition of particulates, failing to burn particulates in a downstream region and thus resulting in difficulty in the regeneration of the honeycomb filter.
Also, when regeneration is carried out by a microwave system as shown in JP 59-126022 A, for instance, a portion of the filter near the air supply side is cooled by air supplied, so that the temperature elevation of particulates is hindered, resulting in difficulty in the burning of particulates and narrowing of a region in which the burning of particulates occurs, and thus difficulty in effective regeneration of the entire honeycomb filter. As a result, when air necessary for the burning of particulates is supplied from an exhaust gas inlet side to repeat the capturing of particulates and regenerating the filter, unburned particulates are so accumulated near an end surface of the filter that flow paths open on an exhaust gas inlet side are clogged, thereby losing a particulates-capturing function and resulting in extreme decrease in a filter-regenerating function.
To solve these problems, JP3-68210B discloses a honeycomb filter having space disposed between plugs positioned on an exhaust gas inlet side and the end surfaces of flow paths on the exhaust gas inlet side. FIG. 4 is a cross-sectional view showing a honeycomb filter 40 described in JP3-68210B. The arrow X indicates an exhaust gas-flowing direction. Because the honeycomb filter of FIG. 4 comprises space 49 between plugs 48a positioned on an upstream side of the flow paths and the flow path inlet ends 41a, particulates in an exhaust gas are captured by partition walls in the space 49 between the plugs 48a on the inlet side and the flow path inlet ends 41a, so that larger amounts of particulates are attached in an upstream region. Accordingly, when particulates are burned by a heating means mounted on the inlet side of the filter, the burning of particulates can be easily conducted in a downstream region.
Japanese Patent 2,924,288 discloses a honeycomb filter-regenerating apparatus comprising a heating chamber mounted onto an exhaust pipe of an engine, a means for generating microwaves that are supplied to the heating chamber, a honeycomb filter contained in the heating chamber for capturing particulates in an exhaust gas, and a means for supplying air to the heating chamber. FIG. 3 is a cross-sectional view showing a honeycomb filter 30 in the honeycomb filter-regenerating apparatus of Japanese Patent 2,924,288. The arrow X indicates an exhaust gas-flowing direction. The honeycomb filter 30 is constituted by a honeycomb structure 31 having a large number of flow paths 37 partitioned by cell walls 36 enclosed by a peripheral wall 35, inlet portions 31a and outlet portions 31b being alternately sealed by plugs 38a, 38b, the plugs 38a being positioned inside the end surfaces of the inlet portion 31a to constitute a heat dissipation-preventing means 39. According to Japanese Patent 2,924,288, when the captured particulates are heated by microwaves, the particulates heated by the heat dissipation-preventing means 39 are prevented from dissipating heat, resulting in increase in a temperature-elevating speed, so that the particulates reach their burning temperature in a short period of time.
In JP3-68210B and Japanese Patent 2,924,288, to conduct regeneration of the entire honeycomb filter efficiently, as shown in FIGS. 3 and 4, the exhaust gas-introducing side plugs are disposed inside the exhaust gas inlet end of the honeycomb filter.
It has been found, however, that when a honeycomb filter having a structure, in which plugs on an exhaust gas inlet side are disposed inside an inlet end of the filter as shown in FIGS. 3 and 4, is actually produced, the following problems occur.
In the honeycomb filter 40 described in JP3-68210B, the plugs 48a on the inlet side are formed as follows: As shown in FIG. 6(a), the end portions of flow paths needing no plugs are sealed with a wax 62, and the inlet end portions 41a of the honeycomb structure 41 are then immersed in a plug-forming material slurry 60 to cause the slurry 60 to enter into the flow paths 47a not sealed by plugs with a wax. Because the honeycomb structure 41 is made of a porous ceramic and thus water-absorptive, an upper portion of the slurry entering into the flow paths 47 is deprived of water by the cell walls and thus solidified, while a lower portion of the slurry remains unchanged for the lack of cell walls that can remove water. This honeycomb structure is turned upside down as shown in FIG. 6(b), so that a slurry remaining in the flow paths spontaneously falls onto the solidified slurry portion to form plugs 48a. The position of the inlet-side plugs is determined by the height of the slurry entering into the cells.
However, the actual trial of the inventors to charge the slurry 60 into the flow paths 47a has revealed that because water is absorbed by the cell walls in contact with the slurry regardless of the position of the slurry, solidification starts simultaneously in the upper and lower portions of the slurry. It is thus difficult to cause solidification only in the upper portion of the slurry, and plugs may be formed up to the ends of the flow passes as shown in FIG. 6(c). Accordingly, it is difficult to provide space as shown in FIGS. 2 and 9-15 of JP3-68210B in cell portions on an upstream side of the inlet-side plugs. This tendency is remarkable, for instance, when the inlet-side plugs are disposed at positions separate from the end surface of the ceramic honeycomb by 10 mm or more. Because it is difficult to surely have space on the upstream side of the plugs on the exhaust gas-introducing side when the honeycomb filter thus formed is actually used to capture particulates, the honeycomb filter cannot exhibit functions of capturing particulates and preventing heat dissipation as expected, failing to efficiently carry out the regeneration of the entire filter, and resulting in a large pressure loss.
In addition, because the degree of solidification of a slurry differs in every flow path, spaces upstream of the inlet-side plugs have different volumes, resulting in uneven pressure loss among the honeycomb filters, and decrease in the production yield of honeycomb filters.
Japanese Patent 2,924,288 fails to disclose a specific method for forming plugs 58a at the inlets 31a. 